Hypothermic cardiopulmonary bypass is commonly used during cardiac surgery. The metabolic and biochemical effects of hypothermia remain poorly understood. Cerebral function during and after cardiopulmonary bypass is an important area of interest in cardiac surgical research. Cerebral dysfunction is a major cause of morbidity and mortality after cardiac operations. The present study was designed to determine the effect of hypothermia on the energy state and the intracellular pH of the brain. Adolescent sheep were studied with 31P NMR spectroscopy during both surface-induced hypothermia and during profound hypothermia produced by cardiopulmonary bypass. Preliminary results show that there is no difference in the brain energy state during hypothermia. The intracellular pH closely paralleled the blood pH. No evidence of cellular acidosis was seen, thus implying that cerebral oxygen delivery was adequate. This study has important clinical implications for hypothermic patients. Cerebral oxygen delivery (as evidenced by preservation of cellular energy charge and lack of acidosis) is adequate both during moderate surface-induced hypothermia and during profound hypothermia produced by cardiopulmonary bypass.